Phase inversion polyetherimide (PEI) flat sheet membranes were surface modified using fluorinated surface modifying macromolecules (SMMs) additives. Two SMM formulations were used. Each SMM was blended into polyetherimide casting solutions containing the solvent N,N-dimethyleacetamide and the non-solvent hydroxybutyric acid gamma-lactone (GBL). The effects of the SMM and the PEI base polymer concentrations on the morphological properties of the prepared membranes have been investigated. Contact angle measurements indicate that PEI membrane surface becomes more hydrophobic after adding the SMMs to the PEI casting solutions, while X-ray photoelectron spectroscopy analysis shows enrichment of fluorine on the PEI membrane surfaces. The SMM modified and unmodified PEI membranes were also characterized by means of atomic force microscopy (AFM), gas permeation tests and ultrafiltration experiments using aqueous solutions of polyethylene glycol (PEG) and polyethylene oxide (PEO) of various molecular weights. Pore sizes and nodule sizes obtained from AFM images were remarkably fitted to the log-normal probability distribution curves. Mean pore sizes, pore size distributions, nodule sizes, nodule size distributions and roughness parameters of the membranes were determined. It was found that SMM actively migrated to the air surface and changed the surface properties of the PEI membranes. The mean pore size and the molecular weight cut-off (MWCO) of the SMM modified PEI membranes were lower than those corresponding to the unmodified membranes, while the nodule sizes were larger. The surface roughness parameters were reduced when SMMs were added.